I'm having the same issue with the exact same model but only when I switch to WPA2PSK security.It starts to happen after 24 hours or so and becomes more frequent. This happens everytime I attempt the switch.

I've had this router for years and it never happens with WEP security but of course that's old and outdated so I don't want to risk it anymore.

I've just updated the firmware to V3.40(APR.5) | 06/17/2010 so let's see what happens.